The present invention relates to loop antenna systems and more particularly to improved devices and techniques for enabling reception of VLF/LF electromagnetic transmissions in seawater.
As is known, the reception capabilities of loop antennas are dependent upon the quality factor Q which is defined as ##EQU1## where the frequency f is in hertz, the inductance L in henries, and the antenna resistance R in ohms. Generally, as the value of Q increases, the reception capabilities of loop antennas exhibit a corresponding increase. In situations where loop antennas are in close proximity to conductive mediums, however, the antennas will magnetically couple to such mediums causing the coupled resistance, which is included in R, to increase and reduce the value of Q. Naturally, as the conductive mediums are removed in distance from the loop antenna, coupled losses will also decrease causing a corresponding increase in Q.
When an insulated loop antenna is submerged in seawater, the antenna quality factor also responds in a manner similar to that described above, with the quality factor increasing as the separation between the antenna and the conductive seawater is increased. Energy coupled from the antenna to the seawater sets up a conduction current in the seawater that flows around the antenna and effectively acts as a shorted turn around the antenna. The shorted turn then acts to dissipate energy that reduces the value of antenna Q as previously described. Since the sensitivity of the antenna to electromagnetic transmissions depends on the maintenance of a certain value of Q, the reduction of the Q value when submerged in seawater seriously affects antenna operation.
In an effort to prevent reduction in antenna sensitivity under submerged conditions, it was necessary in prior known techniques to enclose the antenna in large radome structures to insulate the antenna from the surrounding seawater. As the size of the radomes were increased, a greater separation between seawater and antenna resulted in a larger value of Q and better reception of electromagnetic transmissions. In a particular application to submarine towed watertight communications buoys, the buoy itself served as the radome housing the loop antenna and provided sufficient values of Q to allow acceptable operation. Since the proximity of the seawater determines the Q, however, such radome structures were required to be large (coupled with increased weight) in order to provide acceptable Q. In addition to being very costly, such large radomes were cumbersome and provided increased drag when towed as communications buoys. Further, any attempts to make the communications buoys free flooding, to reduce complexity or cost, correspondingly reduced the antenna Q to an unacceptable value or required the loop antenna to be housed in a separate radome structure. In either case, such techniques were at best limited in flexibility and costly in providing for the use of loop antennas in a seawater environment.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques, and to provide a technique for improving loop antenna reception.